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Definition

Acquired immune deficiency syndrome (AIDS) is an infectious disease caused by the human immunodeficiency virus (HIV). AIDS is the advanced form of infection caused by HIV and typically only manifests itself after a long latency period after initial HIV infection. AIDS is a fatal disease for which there is currently no cure.

Description

First recognized in the United States in 1981, AIDS is considered one of the most devastating public health problems in recent history. The Centers for Disease Control and Prevention (CDC) has estimated that, as of June 2000, between 800,000 and 900,000 people in the United States were HIV-positive, and 312,000 were living with AIDS. Of adult AIDS cases, 47–53% were believed to have contracted HIV from same sex male intercourse, 25–31% from intravenous drug use, and 10% from heterosexual contact. There are an estimated 40,000 new HIV infections each year in the U.S. The Joint United Nations Program on HIV/AIDS estimates that, worldwide during the year 2000, an estimated 3 million people had died of AIDS, and 34.7 million adults and 1.4 million children were living with HIV/AIDS. Approximately 95% of persons with HIV/AIDS were living in developing countries.

Risk factors

HIV/AIDS can be transmitted in several ways. The various routes of transmission (and associated risk factors) include:

• Sexual contact. Persons at greatest risk are those who do not practice safe sex (sex with a condom), those who are not monogamous, those who engage in anal inter-course, and those who have sex with a partner with symptoms of advanced HIV infection and/or other sexually transmitted diseases (STDs). In the United States and Europe, most cases of sexually transmitted HIV infection arise via same sex contact, whereas in Africa the disease is spread primarily through heterosexual intercourse.
• Transmission in pregnancy. High-risk mothers include women who use intravenous drugs, women who have sex with bisexual men, women who are married to men who have an abnormal blood condition called hemophilia (a condition requiring blood transfusions), and women living in areas with a high rate of HIV infection among heterosexuals. The chances of transmitting HIV from mother to child are higher in women who are in advanced stages of the disease. Breast feeding increases the risk of transmission by 10%-20%, and vaginal delivery doubles the risk of transmitting HIV to the child. Zidovudine (AZT) given to the mother during pregnancy and given to the baby soon after delivery have been shown to decrease the risk of HIV transmission to the child.
• Exposure to contaminated blood or blood products. With the introduction of blood product screening in the mid-1980s, the incidence of HIV transmission in blood transfusions has dropped to 1 in 100,000. Among users of intravenous drugs, risk increases with the duration of injection use, the frequency of needle sharing, the number of persons who share a needle, and the number of AIDS cases in the local population.
• Exposure of health care professionals to infected blood. Studies have shown that 0.32% of highly exposed health care workers have become infected with HIV through occupational exposure. Needle injuries are the most common exposure route. Risk factors for contracting HIV from a needle injury include a deep injection, a needle that has been used in an artery or vein, blood visibly apparent on the needle prior to injury, and blood from a patient with end-stage AIDS. There is evidence that administration of zidovudine (AZT) to the injured worker soon after HIV exposure decreases risk of infection.

HIV is not transmitted by handshakes, coughing, sneezing, or other casual non-sexual contact. There is currently no evidence that HIV can be transmitted through bloodsucking insects such as mosquitoes.

AIDS in women

HIV remains an important cause of death and illness in women. In the US, AIDS was the fifth leading cause of death among women aged 25-44 in 1998. In 1999, 32% of new HIV diagnoses were in women. Although HIV infected women have been observed to die earlier than men, it is believed that this difference in survival rates is caused by differences in access to care and delayed treatment rather than biological differences in disease progression.

AIDS in children

Since AIDS can be transmitted from an infected mother to the child during pregnancy, during the birth process, or through breast milk, all infants born to HIV-positive mothers are a high-risk group. In 1999, 78% of new HIV cases in women were in females of childbearing age. Without prenatal intervention, between 20-40% of children born to HIV-positive women will become infected with the virus.

AIDS is one of the 10 leading causes of death in children between one and four years of age. The interval between exposure to HIV and the development of AIDS is shorter in children than in adults. Infants infected with HIV have a 20-30% chance of developing AIDS within a year and dying before age three. In the remainder, AIDS progresses more slowly; the average child patient survives to seven years of age. Some survive into early adolescence.

Causes and symptoms

Because HIV destroys immune system cells, AIDS is a disease that can affect any of the body's major organ systems. HIV attacks the body through three disease processes: immunodeficiency, autoimmunity, and nervous system dysfunction.

Immunodeficiency describes the condition in which the body's immune response is damaged, weakened, or is not functioning properly. In AIDS, immunodeficiency results from the way that the virus binds to a protein called CD4, which is found on the surface of certain subtypes of white blood cells, including helper T cells, macrophages, and monocytes. Once HIV attaches to an immune system cell, it can replicate within the cell and

kill the cell in ways that are still not completely understood. In addition to killing some lymphocytes directly, the AIDS virus disrupts the functioning of the remaining CD4 cells. Because the immune system cells are destroyed, many different types of infections and cancers that take advantage of a person's weakened immune system (opportunistic) can develop.

Autoimmunity is a condition in which the body's immune system produces antibodies that work against its own cells. Antibodies are specific proteins produced in response to exposure to a specific, usually foreign, protein or particle called an antigen. In this case, the body produces antibodies that bind to blood platelets that are necessary for proper blood clotting and tissue repair. Once bound, the antibodies mark the platelets for removal from the body, and they are filtered out by the spleen. Some AIDS patients develop a disorder, called immune-related thrombocytopenia purpura (ITP), in which the number of blood platelets drops to abnormally low levels.

HIV also infects some susceptible cells in the central nervous system. The exact mechanism of HIV entry into the brain is unknown. Possible modes of entry across the blood-brain barrier include HIV entry as a single cell-free viral particle (virion), entry via infected monocyte or lymphocyte, and infection of endothelial cells (cells forming brain border). Regardless of the mechanism, evidence suggests that the cerebral spinal fluid is seeded with HIV very early in the infection process.

Although not all patients will follow them precisely, the course of AIDS generally progresses through the three stages (acute retroviral syndrome, latency period, and late-stage AIDS) that follow.

Acute retroviral syndrome

Acute retroviral syndrome is a term used to describe a group of symptoms that can resemble mononucleosis and that may be the first sign of HIV infection in 50-70% of all patients and 45-90% of women. The symptoms may include fever, fatigue, muscle aches, loss of appetite, digestive disturbances, weight loss, skin rashes, headache, and chronically swollen lymph nodes (lymphadenopathy). Approximately 25-33% of patients will experience a form of meningitis during this phase in which the membranes that cover the brain and spinal cord become inflamed. Acute retroviral syndrome develops between one and six weeks after infection and lasts for two to three weeks. Blood tests during this period will indicate the presence of virus (viremia) and the appearance of the viral p24 antigen in the blood.

Latency period

After the HIV virus enters a patient's lymph nodes during the acute retroviral syndrome stage, the disease becomes latent for as many as 10 years or more before symptoms of advanced disease develop. During latency, the virus continues to replicate in the lymph nodes, where it may cause one or more of the following conditions.

PERSISTENT GENERALIZED LYMPHADENOPATHY (PGL). Persistent generalized lymphadenopathy, or PGL, is a condition in which HIV continues to produce chronic painless swellings in the lymph nodes during the latency period. The lymph nodes that are most frequently affected by PGL are those in the areas of the neck, jaw, groin, and armpits. PGL affects between 50-70% of patients during latency.

CONSTITUTIONAL SYMPTOMS. Many patients will develop low-grade fevers, chronic fatigue, and general weakness. HIV may also cause a combination of food malabsorption, loss of appetite, and increased metabolism that contribute to the so-called AIDS wasting or wasting syndrome.

OTHER ORGAN SYSTEMS. At any time during the course of HIV infection, patients may suffer from a yeast infection in the mouth called thrush, open sores or ulcers, or other infections of the mouth; diarrhea and other gastrointestinal symptoms that cause malnutrition and weight loss; diseases of the lungs and kidneys; and degeneration of the nerve fibers in the arms and legs. HIV infection of the nervous system leads to general loss of strength, loss of reflexes, and feelings of numbness or burning sensations in the feet or lower legs.

Late-stage AIDS

Late-stage AIDS is usually marked by a sharp decline in the number of CD4+ lymphocytes, followed by a rise in the frequency of opportunistic infections and cancers. Doctors monitor the number and proportion of CD4+ lymphocytes in the patient's blood in order to assess the progression of the disease and the effectiveness of different medications. About 10% of infected individuals never progress to this overt stage of the disease.

OPPORTUNISTIC INFECTIONS. Once the patient's CD4+ lymphocyte count falls below 200 cells/mm³, the patient is at risk for a variety of opportunistic infections. The infectious organisms may include the following:

• Fungi. The most common fungal disease associated with AIDS is Pneumocystis carinii pneumonia (PCP). About 70%-80% of AIDS patients will have at least one episode of PCP prior to death. PCP is the immediate cause of death in 15-20% of AIDS patients. It is an important measure of a patient's prognosis. Other fungal infections include a yeast infection of the mouth (candidiasis or thrush) and cryptococcal meningitis.
• Protozoa. Toxoplasmosis is a common opportunistic infection in AIDS patients that is caused by a protozoan. Other diseases in this category include amebiasis and cryptosporidiosis.
• Mycobacteria. AIDS patients may develop tuberculosis or MAC infections. MAC infections are caused by Mycobacterium avium-intracellulare, and occur in about 40% of AIDS patients.
• Bacteria. AIDS patients are likely to develop bacterial infections of the skin and digestive tract.
• Viruses. AIDS patients are highly vulnerable to cytomegalovirus (CMV), herpes simplex virus (HSV), varicella zoster virus (VZV), and Epstein-Barr virus (EBV) infections. Another virus, JC virus, causes progressive destruction of brain tissue in the brain stem, cerebrum, and cerebellum (multifocal leukoencephalopathy or PML), which is regarded as an AIDS-defining illness by the Centers for Disease Control and Prevention.

AIDS DEMENTIA COMPLEX AND NEUROLOGIC COMPLICATIONS. AIDS dementia complex is a late complication of the disease. It is unclear whether it is caused by the direct effects of the virus on the brain or by intermediate causes. AIDS dementia complex is marked by loss of reasoning ability, loss of memory, inability to concentrate, apathy and loss of initiative, and unsteadiness or weakness in walking. Some patients also develop seizures. There are no specific treatments for AIDS dementia complex.

MUSCULOSKELETAL COMPLICATIONS. Patients in late-stage AIDS may develop inflammations of the muscles, particularly in the hip area, and may have arthritis- like pains in the joints.

ORAL SYMPTOMS. In addition to thrush and painful ulcers in the mouth, patients may develop a condition called hairy leukoplakia of the tongue. This condition is also regarded by the CDC as an indicator of AIDS. Hairy leukoplakia is a white area of diseased tissue on the tongue that may be flat or slightly raised. It is caused by the Epstein-Barr virus.

AIDS-RELATED CANCERS. Patients with late-stage AIDS may develop Kaposi's sarcoma (KS), a skin tumor that primarily affects homosexual men. KS is the most common AIDS-related malignancy. It is characterized by reddish-purple blotches or patches (brownish in persons with darker skin) on the skin or in the mouth. About 40% of patients with KS develop symptoms in the digestive tract or lungs. KS may be caused by a herpes virus-like sexually transmitted disease agent rather than HIV.

The second most common form of cancer in AIDS patients is a tumor of the lymphatic system (lymphoma). AIDS-related lymphomas often affect the central nervous system and develop very aggressively.

Invasive cancer of the cervix is an important diagnostic marker of AIDS in women.

Diagnosis

Because HIV infection produces such a wide range of symptoms, the CDC has drawn up a list of 34 conditions regarded as defining AIDS. The physician will use the CDC list to decide whether the patient falls into one of these three groups:

• definitive diagnoses with or without laboratory evidence of HIV infection
• definitive diagnoses with laboratory evidence of HIV infection
• presumptive diagnoses with laboratory evidence of HIV infection

Physical findings

Almost all the symptoms of AIDS can occur with other diseases. The general physical examination may range from normal findings to symptoms that are closely associated with AIDS. These symptoms are hairy leukoplakia of the tongue and Kaposi's sarcoma. When the doctor examines the patient, he or she will look for the overall pattern of symptoms rather than any one finding.

Laboratory tests for HIV infection

BLOOD TESTS (SEROLOGY). The first blood test for AIDS was developed in 1985. At present, patients who are being tested for HIV infection are usually given an enzyme-linked immunosorbent assay (ELISA) test for the presence of HIV antibody in their blood. Positive ELISA results are then tested with a Western blot or immunofluorescence (IFA) assay for confirmation. The combination of the ELISA and Western blot tests is more than 99.9% accurate in detecting HIV infection within four to eight weeks following exposure. Indeterminate test results are possible (positive ELISA but non-confirmatory Western blot result) if the tests are given within the window period after infection (up to eight weeks after infection, but may be longer). In these indeterminate cases, the ELISA and Western blot should be repeated every three months until a definitive result is made. The patient should be considered HIV positive until proven otherwise. The polymerase chain reaction (PCR) test can be used to detect the presence of viral nucleic acids in the very small number of HIV patients who have false-negative results on the ELISA and Western blot tests.

OTHER LABORATORY TESTS. In addition to diagnostic blood tests, other blood tests are used to track the course of AIDS in patients that have already been diagnosed, including blood counts, viral load tests, p24 antigen assays, and measurements of [.beta]₂^-microglobulin ([.beta]₂[.Mu]).

Doctors will use a wide variety of tests to diagnose the presence of opportunistic infections, cancers, or other disease conditions in AIDS patients. Tissue biopsies, samples of cerebrospinal fluid, and sophisticated imaging techniques, such as magnetic resonance imaging (MRI) and computed tomography scans (CT) are used to diagnose AIDS-related cancers, some opportunistic infections, damage to the central nervous system, and wasting of the muscles. Urine and stool samples are used to diagnose infections caused by parasites. AIDS patients are also given blood tests for syphilis and other sexually transmitted diseases.

Diagnosis in children

Diagnostic blood testing in children older than 18 months is similar to adult testing, with ELISA screening confirmed by Western blot. Younger infants can be diagnosed by direct culture of the HIV virus, PCR testing, and p24 antigen testing.

In terms of symptoms, children are less likely than adults to have an early acute syndrome. They are, however, likely to have delayed growth, a history of frequent illness, recurrent ear infections, a low blood cell count, failure to gain weight, and unexplained fevers. Children with AIDS are more likely to develop bacterial infections, inflammation of the lungs, and AIDS-related brain disorders than are HIV-positive adults.

Treatment

Because AIDS is a fatal disease, AIDS therapies focus on improving the quality and length of life for AIDS patients by slowing or halting the replication of the virus, and treating or preventing infections and cancers that take advantage of a person's weakened immune system. No vaccine is effective in preventing HIV infection.

Treatment for AIDS covers four considerations:

TREATMENT OF OPPORTUNISTIC INFECTIONS AND MALIGNANCIES. Most AIDS patients require complex long-term treatment with medications for infectious diseases. This treatment is often complicated by the development of resistance in the disease organisms. AIDS-related malignancies in the central nervous system are usually treated with radiation therapy. Cancers elsewhere in the body are treated with chemotherapy.

PROPHYLACTIC TREATMENT FOR OPPORTUNISTIC INFECTIONS. Prophylactic treatment is treatment that is given to prevent disease. AIDS patients with a history of Pneumocystis pneumonia; with CD4+ counts below 200

cells/mm³ or 14% of lymphocytes; weight loss; or thrush should be given prophylactic medications. The three drugs given are trimethoprim-sulfamethoxazole, dapsone, or pentamidine in aerosol form.

ANTI-RETROVIRAL TREATMENT. In recent years researchers have developed drugs that suppress HIV replication, as distinct from treating its effects on the body. These drugs fall into three classes:

• Nucleoside reverse transcriptase inhibitors (NRTIs). These drugs work by looking very similar to the molecules acted upon by the HIV enzyme reverse transcriptase. Reverse transcriptase binds to these drugs, which in turn stop the viral replication process. These drugs include zidovudine, didanosine (ddi), zalcitabine (ddC), stavudine (d4T), lamivudine (3TC), and abacavir (ABC).
• Non-nucleoside reverse transcriptase inhibitors (NNRTIs). These drugs de-activate the HIV enzyme reverse transcriptase. This class of drugs includes nevirapine (NVP), delavirdine (DLV), and efavirenz (EFV).

Protease inhibitors. A new class of drugs, protease inhibitors are effective against HIV strains that have developed resistance to nucleoside analogues and are used in combination with them. These compounds include saquinavir (SQV), ritonavir (RJV), indinavir (IDV), nelfinavir (NFV), and amprenavir (APV).

New combinations of therapies are also being developed, primarily to improve adherence. Trizivir for the treatment of HIV in adults and adolescents is a fixeddose

Risk of acquiring HIV infection by entry site
Entry site	Risk virus reaches entry site	Risk virus enters	Risk inoculated
SOURCE: Hopp, J.W. and E.A. Rogers. AIDS and the Allied Health Professions. Philadelphia: F.A. Davis Co., 1989.
Conjuntiva	Moderate	Moderate Very low
Oral mucosa	Moderate	Moderate	Low
Nasal mucosa	Low	Low	Very low
Lower respiratory	Very low	Very low	Very low
Anus	Very high	Very high	Very high
Skin, intact	Very low	Very low	Very low
Skin, broken	Low	High	High
Sexual:
Vagina	Low Low	Medium
Penis	High	Low	Low
Ulcers (STD)	High	High	Very high
Blood:
Products	High	High	High
Shared needles	High	High	Very high
Accidental needle	Low	High	Low
Traumatic wound	Modest	High	High
Perinatal	High	High	High

combination of abacavir, zidovudine, and lamivudine. Another combination therapy, Combivir, combines lamivudine and zidovudine. Both Trizivir and Combivir are combinations of NRTIs that combine drugs into a single dosage, making it easier for patients to comply with their dosage regimens.

Treatment guidelines for these agents are continually being modified as new medications are developed and introduced. Guidelines for when to start anti-retroviral therapy have been published separately by the International AIDS Society—United States and U.S. Department of Health and Human Services. These guidelines are very similar and base their recommendations on a patient's CD4 counts, viral load, and clinical symptoms.

In terms of specific treatment approaches, the January 2000 guidelines from the U.S. Department of Health and Human Services suggest two strategies for initial treatment, both of which use combinations of drugs: two nucleosides and a protease inhibitor, or two nucleosides and a non-nucleoside drug. Over time, treatment changes may be required; factors that must be considered when changing treatment regimens include drug toxicity, clinical symptoms, viral load, CD4 counts, adherence to current and future medications, and other viable treatment options.

STIMULATION OF BLOOD CELL PRODUCTION. Because many patients with AIDS suffer from abnormally low levels of both red and white blood cells, they may be given medications to stimulate blood cell production. Epoetin alfa (erythropoietin) may be given to anemic patients. Patients with low white blood cell counts may be given filgrastim or sargramostim.

Treatment in women

Treatment of pregnant women with HIV is particularly important because anti-retroviral therapy has been shown to reduce transmission to the infant by 65%.

Prognosis

No cure for AIDS has been discovered. Treatment stresses aggressive combination drug therapy for those patients with access to the expensive medications and who tolerate them adequately. The use of these multi-drug therapies, called highly active antiretroviral therapies or HAART, has significantly reduced the numbers of deaths in the United states resulting from AIDS. The data is still inconclusive, but the potential exists to prolong life indefinitely using these and other drug therapies to boost the immune system, keep the virus from replicating, and ward off opportunistic infections and malignancies.

Prognosis after the latency period depends on the patient's specific symptoms and the organ systems affected by the disease. Patients with AIDS-related lymphomas of the central nervous system die within two to three months of diagnosis; those with systemic lymphomas may survive for eight to ten months.

Health care team roles

The physician oversees the treatment strategy and patient evaluation for patients who are HIV-positive and/or have AIDS. Adherence to treatment is a critical aspect of clinical care in AIDS, and nurses play a key role in educating patients and providing them with adherence tools. Nurses, social workers, and psychologists can also be trained as HIV counselors to advise patients about HIV testing and, if necessary, to assist and guide patients in adjusting to a life with HIV. During end-stage AIDS, nurses, social workers, and other hospice workers ensure that patients do not experience unnecessary pain and discomfort.

Prevention

As of 2000, there is no vaccine effective against HIV/AIDS. Several vaccines are being investigated, however, both to prevent initial HIV infection and as a therapeutic treatment to prevent HIV from progressing to full-blown AIDS.

Several types of prevention programs have been found to be effective in reducing sexual transmission of HIV. These include:

• targeted education for at-risk groups, emphasizing preventive practices such as condom use, monogamy, and HIV testing prior to beginning a sexual relationship
• counseling with or without testing for HIV and other sexually transmitted diseases
• education programs in institutions such as the military, prisons, and the workplace
• greater access to condoms

Preventive measures for other modes of transmission include:

• Making clean needles more available and discouraging intravenous drug users from sharing needles.
• Encouraging health care professionals to take all necessary precautions by wearing gloves and masks when handling body fluids.
• Encouraging health care institutions to provide safer medical devices such as self-sheathing needles and retracting and/or needleless intravenous systems.
• Informing individuals who are planning to undergo major surgery that they can donate blood in advance to prevent a risk of infection from a blood transfusion. (However, blood and blood products are carefully monitored.)
• Encouraging testing for HIV infection if there has been suspected exposure to HIV. If HIV infection is confirmed, sexual partners should be informed and, if necessary, receive medical attention.

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KEY TERMS

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Acute retroviral syndrome—A group of symptoms resembling mononucleosis that often are the first sign of HIV infection in 50-70% of all patients and 45-90% of women.

AIDS dementia complex—A type of brain dysfunction caused by HIV infection that causes difficulty thinking, confusion, and loss of muscular coordination.

Antibody—A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.

Antigen—Any substance that stimulates the body to produce antibody.

Autoimmunity—A condition in which the body's immune system produces antibodies in response to its own tissues or blood components instead of foreign particles or microorganisms.

CD4—A type of protein molecule in human blood, sometimes called the T4 antigen, that is present on the surface of 65% of immune cells. The HIV virus infects cells that have CD4 surface proteins, and as a result, depletes the number of T cells, B cells, natural killer cells, and monocytes in the patient's blood. Most of the damage to an AIDS patient's immune system is done by the virus' destruction of CD4+ lymphocytes.

Hairy leukoplakia of the tongue—A white area of diseased tissue on the tongue that may be flat or slightly raised. It is caused by the Epstein-Barr virus and is an important diagnostic sign of AIDS.

Hemophilia—Any of several hereditary blood coagulation disorders occurring almost exclusively in males. Because blood does not clot properly, even minor injuries can cause significant blood loss that may require a blood transfusion, with its associated minor risk of infection.

Human immunodeficiency virus (HIV)—A transmissible retrovirus that causes AIDS in humans. Two forms of HIV are now recognized: HIV-1, which causes most cases of AIDS in Europe, North and South America, and most parts of Africa; and HIV-2, which is chiefly found in West African patients. HIV-2, discovered in 1986, appears to be less virulent than HIV-1 and may also have a longer latency period.

Immunodeficient—A condition in which the body's immune response is damaged, weakened, or is not functioning properly.

Kaposi's sarcoma—A cancer of the connective tissue that produces painless purplish red (in people with light skin) or brown (in people with dark skin) blotches on the skin. It is a major diagnostic marker of AIDS

Latent period—Also called incubation period, the time between infection with a disease-causing agent and the development of disease.

Lymphocyte—A type of white blood cell that is important in the formation of antibodies and that can be used to monitor the health of AIDS patients.

Lymphoma—A cancerous tumor in the lymphatic system that is associated with a poor prognosis in AIDS patients.

Macrophage—A large white blood cell, found primarily in the bloodstream and connective tissue, that helps the body fight off infections by ingesting the disease-causing organism. HIV can infect and kill macrophages.

Monocyte—A large white blood cell that is formed in the bone marrow and spleen. About 4% of the white blood cells in normal adults are monocytes.

Mycobacterium avium (MAC) infection—A type of opportunistic infection that occurs in about 40% of AIDS patients and is regarded as an AIDS-defining disease.

Non-nucleoside reverse transcriptase inhibitors— A newer class of anti-retroviral drugs that work by inhibiting the reverse transcriptase enzyme necessary for HIV replication.

Nucleoside analogue reverse transcriptase inhibitors—The first group of effective anti-retroviral medications. They work by interfering with HIV synthesis of its viral DNA.

Opportunistic infection—An infection by organisms that usually do not cause infection in people with healthy functioning immune systems.

Persistent generalized lymphadenopathy (PGL)—A condition in which HIV continues to produce chronic painless swellings in the lymph nodes during the latency period.

Pneumocystis carinii pneumonia (PCP)—An opportunistic infection caused by a fungus that is a major cause of death in patients with late-stage AIDS.

Progressive multifocal leukoencephalopathy (PML)—A disease caused by a virus that destroys white matter in localized areas of the brain. It is regarded as an AIDS-defining illness.

Protease inhibitors—A new class of anti-retroviral drugs used to treat AIDS that works by preventing the HIV protease enzyme from generating new functioning HIV viruses.

Protozoan—A single-celled, usually microscopic organism that is eukaryotic and, therefore, different from bacteria (prokaryotic).

Retrovirus—A virus that contains a unique enzyme called reverse transcriptase that allows it to replicate within new host cells.

T cells—Lymphocytes that originate in the thymus gland. T cells regulate the immune system's response to infections, including HIV. CD4 lymphocytes are a subset of T lymphocytes.

Thrush—A yeast infection of the mouth characterized by white patches on the inside of the mouth and cheeks.

Viremia—The measurable presence of virus in the bloodstream that is a characteristic of acute retroviral syndrome.

Wasting syndrome—A progressive loss of weight and muscle tissue caused by the AIDS virus.

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Genevieve Pham-Kanter